1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to optical modulation and detection in coherent optical transmission systems.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
There is a continued quest to improve receiver sensitivity in optical communication systems, particularly for free-space optical communications. Improving receiver sensitivity or reducing the required signal photons per bit (ppb) directly leads to improved transmission link performance. Receiver sensitivity is determined primarily by the modulation format and detection scheme utilized. M-ary pulse-position-modulation (m-PPM) is a well-established modulation format for achieving high receiver sensitivity. m, the size of the alphabet, is usually a power of two so that each symbol represents log2m bits.
When used with ideal photon-counting receivers, m-PPM is capable of approaching the Shannon limit with the increase of m. However, photon-counting receivers currently have limited bandwidth and are not suitable for high-speed (>1 Gb/s) optical transmission. When used with optical amplified receivers, the theoretical sensitivity of m-PPM becomes lower than when an ideal photon-counting receiver is utilized. In addition, the use of large m in m-PPM reduces the channel data rate for a given (slot) modulation speed. M-ary frequency-shift keying (m-FSK) is a frequency-domain equivalent of m-PPM, and has the same theoretical performance as m-PPM. The identification of the modulated frequency carriers in a m-FSK signal can be realized through either optical filters before detection or digital filters after digital coherent detection. M-FSK advantageously has a constant power profile in comparison to m-PPM. It is known that m-PPM has high peak-to-average-power ratio (PAPR) for large m and may encounter fiber nonlinearity issues, e.g., in optical booster amplifiers. So, m-FSK may be more attractive than m-PPM for large m values when the PAPR issue becomes a concern. M-FSK can be implemented with high spectral efficiency by using a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) like frequency grid, where the minimum spacing between possible carriers is 1/T, and T is the time duration of the carrier in each m-FSK symbol. Recently, direct-detection binary differential phase-shift keying (DPSK) and coherent-detection binary phase-shift keying (BPSK) have been used to achieve high-sensitivity and high-data-rate communication, but their sensitivities are not as high as those achieved with high-level m-PPM.